I. Field of the Invention
The present invention relates to communication systems, particularly to a method and apparatus for performing handoff between two sectors of a common base station.
II. Description of the Related Art
In a code division multiple access (CDMA) cellular telephone, wireless local loop or personal communications system, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a remote unit and more than one base station. Signals occupying the common frequency band are discriminated at the receiving station through the spread spectrum CDMA waveform properties based on the use of a high speed pseudonoise (PN) code. The high speed PN code is used to modulate signals transmitted from both the base stations and the remote units. Transmitter stations using different PN codes or PN codes that are offset in time produce signals that can be separately received at the receiving station. The high speed PN modulation also allows the receiving station to receive several instances of a common signal from a single transmitting station where the signal has traveled over several distinct propagation paths due to the multipath characteristics of the radio channel or purposefully introduced diversity.
The multipath characteristics of the radio channel create multipath signals which travel several distinct propagation paths between the transmitting station and the receiving station. One characteristic of a multipath channel is the time spread introduced in a signal that is transmitted through the channel. For example, if an ideal impulse is transmitted over a multipath channel, the received signal appears as a stream of pulses. Another characteristic of the multipath channel is that each path through the channel may cause a different attenuation factor. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different signal strength than the other received pulses. Yet another characteristic of the multipath channel is that each path through the channel may cause a different phase on the signal. For example, if an ideal impulse is transmitted over a multipath channel, each pulse of the received stream of pulses generally has a different phase than the other received pulses.
In the radio channel, the multipath is created by reflection of the signal from obstacles in the environment, such as buildings, trees, cars, and people. In general the radio channel is a time varying multipath channel due to the relative motion of the structures that create the multipath. For example, if an ideal impulse is transmitted over the time varying multipath channel, the received stream of pulses would change in time, location, attenuation, and phase as a function of the time that the ideal impulse is transmitted.
The multipath characteristics of a channel can cause signal fading. Fading is the result of the phasing characteristics of the multipath channel. A fade occurs when multipath vectors act destructively, yielding a received signal that is smaller than either individual vector. For example if a sine wave is transmitted through a multipath channel having two paths where the first path has an attenuation factor of X dB (decibels), a time delay of .delta. with a phase shift of .THETA. radians, and the second path has an attenuation factor of X dB, a time delay of .delta. with a phase shift of .THETA.+.pi. radians, no signal would be received at the output of the channel.
In narrow band modulation systems such as the analog FM modulation employed by conventional radio telephone systems, the existence of multiple path in the radio channel results in severe multipath fading. As noted above with a wideband CDMA, however, the different paths may be discriminated at the receiving station in the demodulation process. The discrimination of multipath signals not only greatly reduces the severity of multipath fading but provides an advantage to the CDMA system.
In an exemplary CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the remote unit is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The remote unit is equipped with a searching element that allows the remote unit to track the signal strength of the pilot signal from a group of base stations including the neighboring base stations.
A method and system for providing a communication with a remote unit through more than one base station during the handoff process are disclosed in U.S. Pat. No. 5,267,261, entitled "MOBILE ASSISTED SOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM," issued Nov. 30, 1993 assigned to the assignee of the present invention. Using this system, communication between the remote unit and the end user is uninterrupted by the eventual handoff from an original base station to a subsequent base station. This type of handoff may be considered as a "soft" handoff in that communication with the subsequent base station is established before communication with the original base station is terminated. When the remote unit is in communication with two base stations, the remote unit combines the signals received from each base station in the same manner that multipath signals from a common base station are combined.
In a typical macrocellular system, a system controller may be employed to create a single signal for the other end user from the signals received by each base station. Within each base station, signals received from a common remote unit may be combined before they are decoded and thus take full advantage of the multiple signals received. The decoded result from each base station is provided to the system controller. Once a signal has been decoded it cannot be `combined` with other signals. Thus, the system controller must select between the plurality of decoded signals produced by each base station with which communication is established by a single remote unit. The most advantageous decoded signal is selected from among the base station and the other signals are simply discarded.
Remote unit assisted soft handoff operates based on the pilot signal strength of several sets of base stations as measured by the remote unit. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability of having a signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength at a sufficient signal level to establish communication.
When communications are initially established, a remote unit communicates through a first base station and the Active Set contains only the first base station. The remote unit monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the remote unit. The remote unit communicates a message to the first base station identifying the new base station. A cellular or personal communication system controller decides whether to establish communication between the new base station and the remote unit. Should the cellular or personal communication system controller decide to do so, the cellular or personal communication system controller sends a message to the new base station with identifying information about the remote unit and a command to establish communications therewith. A message is also transmitted to the remote unit through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The remote unit searches for the new base station transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations.
When the remote unit is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the remote unit generates and transmits a message to report the event. The cellular or personal communication system controller receives this message through at least one of the base stations with which the remote unit is communicating. The cellular or personal communication system controller may decide to terminate communications through the base station having a weak pilot signal strength.
The cellular or personal communication system controller upon deciding to terminate communications through a base station generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established sends a message to the remote unit. The cellular or personal communication system controller also communicates information to the base station to terminate communications with the remote unit. The remote unit communications are thus routed only through base stations identified in the new Active Set.
Because the remote unit is communicating with the end user though at least one base station at all times throughout the soft handoff process, no interruption in communication occurs between the remote unit and the end user. A soft handoff provides significant benefits in its inherent "make before break" communication over conventional "break before make" techniques employed in other cellular communication systems.
In a cellular or personal communication telephone system, maximizing the capacity of the system in terms of the number of simultaneous telephone calls that can be handled is extremely important. System capacity in a spread spectrum system can be maximized if the transmission power of each remote unit is controlled such that each transmitted signal arrives at the base station receiver at the same level. In an actual system, each remote unit may transmit the minimum signal level that produces a signal-to-noise ratio that allows acceptable data recovery. If a signal transmitted by a remote unit arrives at the base station receiver at a power level that is too low, the bit-error-rate may be too high to permit high quality communications due to interference from the other remote units. On the other hand, if the remote unit transmitted signal is at a power level that is too high when received at the base station, communication with this particular remote unit is acceptable but this high power signal acts as interference to other remote units. This interference may adversely affect communications with other remote units.
Therefore, to maximize capacity in an exemplary CDMA spread spectrum system, the transmit power of each remote unit within the coverage area of a base station is controlled by the base station to produce the same nominal received signal power at the base station. In the ideal case, the total signal power received at the base station is equal to the nominal power received from each remote unit multiplied by the number of remote units transmitting within the coverage area of the base station plus the power received at the base station from remote units in the coverage area of neighboring base stations.
The path loss in the radio channel can be characterized by two separate phenomena: average path loss and fading. The forward link, from the base station to the remote unit, operates on a different frequency than the reverse link, from the remote unit to the base station. However because the forward link and reverse link frequencies are within the same general frequency band, a significant correlation between the average path loss of the two links exists. On the other hand, fading is an independent phenomenon for the forward link and reverse link and varies as a function of time.
In an exemplary CDMA system, each remote unit estimates the path loss of the forward link based on the total power at the input to the remote unit. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the remote unit. From the estimate of the average forward link path loss, the remote unit sets the transmit level of the reverse link signal. Should the reverse link channel for one remote unit suddenly improve compared to the forward link channel for the same remote unit due to independent fading of the two channels, the signal as received at the base station from this remote unit would increase in power. This increase in power causes additional interference to all signals sharing the same frequency assignment. Thus, a rapid response of the remote unit transmit power to the sudden improvement in the channel would improve system performance. Therefore, it is necessary to have the base station continually contribute to the power control mechanism of the remote unit.
Remote unit transmit power may also be controlled by one or more base stations. Each base station with which the remote unit is in communication measures the received signal strength from the remote unit. The measured signal strength is compared to a desired signal strength level for that particular remote unit. A power adjustment command is generated by each base station and sent to the remote unit on the forward link. In response to the base station power adjustment command, the remote unit increases or decreases the remote unit transmit power by a predetermined amount. By this method, a rapid response to a change in the channel is effected and the average system performance is improved. Note that in a typical cellular system, the base stations are not intimately connected and each base station in the system is unaware of the power level at which the other base stations receive the remote unit's signal.
When a remote unit is in communication with more than one base station, power adjustment commands are provided from each base station. The remote unit acts upon these multiple base station power adjustment commands to avoid transmit power levels that may adversely interfere with other remote unit communications and yet provide sufficient power to support communication from the remote unit to at least one of the base stations. This power control mechanism is accomplished by having the remote unit increase its transmit signal level only if every base station with which the remote unit is in communication requests an increase in power level. The remote unit decreases its transmit signal level if any base station with which the remote unit is in communication requests that the power be decreased. A system for base station and remote unit power control is disclosed in U.S. Pat. No. 5,056,109 entitled "METHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE TELEPHONE SYSTEM," issued Oct. 8, 1991, assigned to the Assignee of the present invention.
Base station diversity at the remote unit is an important consideration in the soft handoff process. The power control method described above operates optimally when the remote unit communicates with each base station through which communication is possible. In doing so, the remote unit avoids inadvertently interfering with communications through a base station receiving the remote unit's signal at an excessive level but unable to communicate a power adjustment command to the remote unit because communication is not established therewith.
A typical cellular or personal communication system contains some base stations having multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas. The process of simultaneous communication with two sectors of the same base station is called softer handoff. The process of soft handoff and the process of softer handoff are the same from the remote unit's perspective. However, the base station operation in softer handoff is different from soft handoff. When a remote unit is communicating with two sectors of the same base station, the demodulated data signals of both sectors are available for combination within the base station before the signals are passed to the cellular or personal communication system controller. Because the two sectors of a common base station share circuitry and controlling functions, a variety of information is readily available to sectors of a common base station that is not available between independent base stations. Also two sectors of a common base station send the same power control information to a remote unit (as discussed below).
The combination process in softer handoff allows demodulated data from different sectors to be combined before decoding and thus produce a single soft decision output value. The combination process can be performed based on the relative signal level of each signal thus providing the most reliable combination process.
As noted above, the base station can receive multiple instances of the same remote unit signal. Each demodulated instance of the arriving signal is assigned to a demodulation element. The demodulated output of the demodulation element is combined. The combined signal is decoded. The demodulation elements, instead of being assigned to a single sector, may be assigned to a signal from any one of a set of sectors in the base station. Thus, the base station may use it resources with high efficiency by assigning demodulation elements to the strongest signals available.
Combining signals from sectors of a common base station also allows a sectorized base station to make a single power adjustment command for remote unit power control. Thus, the power adjustment command from each sector of a common base station is the same. This uniformity in power control allows flexible handoff operation in that sector diversity at the remote unit is not critical to the power control process. Further details of the softer handoff process are disclosed in U.S. patent application Ser. No. 08/144,903, filed Oct. 30, 1993, entitled "METHOD AND APPARATUS FOR PERFORMING HANDOFF BETWEEN SECTORS OF A COMMON BASE STATION," assigned to the assignee of the present invention. Further information on the benefits and application of softer handoff are disclosed in U.S. patent application Ser. No. 08/144,901, filed Oct. 30, 1993, entitled "METHOD AND APPARATUS FOR REDUCING THE AVERAGE TRANSMIT POWER FROM A SECTORIZED BASE STATION" and U.S. patent application Ser. No. 08/316,155, filed Sep. 30, 1994 entitled "METHOD AND APPARATUS FOR REDUCING THE AVERAGE TRANSMIT POWER OF A BASE STATION" each of which is assigned to the assignee of the present invention.
Each base station in the cellular system has a forward link coverage area and a reverse link coverage area. These coverage areas define the physical boundary beyond which base station communication with a remote unit is degraded. In other words, if a remote unit is within the base station's coverage area, the remote unit can communicate with the base station, but if the remote unit is beyond the coverage area, communications are compromised. A base station may have single or multiple sectors. Single sectored base stations have approximately a circular coverage area. Multi-sectored base stations have independent coverage areas that form lobes radiating from the base station.
Base station coverage areas have two handoff boundaries. A handoff boundary is defined as the physical location between two base stations where the link would perform the same regardless of whether the remote unit were communicating with the first or second base station. Each base station has a forward link handoff boundary and a reverse link handoff boundary. The forward link handoff boundary is defined as the location where the remote unit's receiver would perform the same regardless of which base station it was receiving. The reverse link handoff boundary is defined as the location of the remote unit where two base station receivers would perform the same with respect to that remote unit.
Ideally these boundaries should be balanced, meaning that they should have the same physical location. If they are not balanced, system capacity may be reduced as the power control process is disturbed or the handoff region unreasonably expands. Note that handoff boundary balance is a function of time, in that the reverse link coverage area shrinks as the number of remote units present therein increases. Reverse link power, which increases with each additional remote unit, is inversely proportional to the reverse link coverage area. An increase in receive power decreases the effective size of the reverse link coverage area of the base station and causes the reverse link handoff boundary to move inward toward the base station.
To obtain high performance in a CDMA or other cellular system, it is important to carefully and accurately control the transmit power level of the base stations and remote units in the system. Transmit power control limits the amount of self-interference produced by the system. Moreover, on the forward link, a precise level of transmit power can serve to balance the forward and reverse link handoff boundaries of a base station or a single sector of a multi-sectored base station. Such balancing helps to reduce the size of the handoff regions, increase overall system capacity, and improve remote unit performance in the handoff region.
Before adding a new base station to the existing network, the forward link (i.e. transmit) power and the reverse link (i.e. receive) signal power of the new base station are both approximately equal to zero. To begin the process of adding the new base station, an attenuator in the receive path of the new base station is set to a high attenuation level, creating a high level of artificial noise receive power. An attenuator in the transmit path is also set to a high attenuation level, which in turn causes a low transmit power level. The high level of artificial noise receive power results in the reverse link coverage area of the new base station being very small. Similarly, because the forward link coverage area is directly proportional to the transmit power, the very low transmit power level and the forward link coverage area is also very small.
The process then continues by adjusting the attenuators in the receive and transmit paths in unison. The attenuation level of the attenuator in the receive path is decreased, thereby decreasing the level of artificial noise receive power, increasing the natural signal level, and hence increasing the size of the reverse link coverage area. The attenuation level of the transmit path attenuator is also decreased, thereby increasing the transmit power level of the new base station and expanding its forward link coverage area. The rate at which the transmit power is increased and the artificial noise receive power is decreased must be sufficiently slow to permit handoff of calls between the new and surrounding base stations as the new base station is added to or removed from the system.
Each base station in the system is initially calibrated such that the sum of the unloaded receiver path noise and the desired pilot power is equal to some constant. The calibration constant is consistent throughout the system of base stations. As the system becomes loaded (i.e. remote units begin to communicate with the base stations), a compensation network maintains the constant relationship between the reverse link power received at the base station and the pilot power transmitted from the base station. The loading of a base station effectively moves the reverse link handoff boundary closer in toward the base station. Therefore, to imitate the same effect on the forward link, the pilot power is decreased as loading is increased. The process of balancing the forward link handoff boundary to the reverse link handoff boundary is referred to as base station breathing is detailed in U.S. Pat. No. 5,548,812 entitled "METHOD AND APPARATUS FOR BALANCING THE FORWARD LINK HANDOFF BOUNDARY TO THE REVERSE LINK HANDOFF BOUNDARY IN A CELLULAR COMMUNICATION SYSTEM" issued Aug. 20, 1996 and assigned to the assignee of the present invention. The process of balancing the forward link handoff boundary to the reverse link handoff boundary during the addition or removal of a base station from a system is referred to as base station blossoming and wilting is detailed in U.S. Pat. No. 5,475,870 entitled "APPARATUS AND METHOD FOR ADDING AND REMOVING A BASE STATION FROM A CELLULAR COMMUNICATION SYSTEM" issued Dec. 12, 1995 and assigned to the assignee of the present invention.
It is desirable to control the relative power used in each forward link signal transmitted by the base station in response to control information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. An example of such a location is a point where the path loss to one or two neighboring base stations is nearly the same as the path loss to the base station communicating with the remote unit. In such a location, the total interference would be increased by three times over the interference seen by a remote unit at a point relatively close to its base station. In addition, the interference coming from the neighboring base stations does not fade in unison with the signal from the active base station as would be the case for interference coming from the active base station. A remote unit in such a situation may require 3 to 4 dB of additional signal power from the active base station to achieve adequate performance.
At other times, the remote unit may be located where the signal-to-interference ratio is unusually good. In such a case, the base station could transmit the corresponding forward link signal using a lower than nominal transmitter power, reducing interference to other signals being transmitted by the system.
To achieve the above objectives, a signal-to-interference measurement capability can be provided within the remote unit receiver. A signal-to-interference measurement is performed by comparing the power of the desired signal to the total interference and noise power. If the measured ratio is less than a predetermined value, the remote unit transmits a request to the base station for additional power on the forward link. If the ratio exceeds the predetermined value, the remote unit transmits a request for power reduction. One method by which the remote unit receiver can monitor signal-to-interference ratios is by monitoring the frame error rate (FER) of the resulting signal.
The base station receives the power adjustment requests from each remote unit and responds by adjusting the power allocated to the corresponding forward link signal by a predetermined amount. The adjustment would usually be small, typically on the order of 0.5 to 1.0 dB, or around 12%. The rate of change of power may be somewhat slower than that used for the reverse link, perhaps once per second. In the preferred embodiment, the dynamic range of the forward link adjustment is typically limited such as from 4 dB less than nominal to about 6 dB greater than nominal transmit power.
CDMA base stations have the ability to provide accurate control over their transmit power level. To provide accurate power control, it is necessary to compensate for variations in the gain in the various components comprising the transmit chain of the base station. Variations in the gain typically occur over temperature and aging such that a simple calibration procedure at deployment does not guarantee a precise level of output transmit power over time. Variations in the gain can be compensated by adjusting the overall gain in the transmit chain so that the actual transmit power of the base station matches a calculated desired transmit power. Each base station sector produces several signaling channels which operate at a variety of data rates and relative signal levels which combined create a raw radio frequency transmit signal. The channel element modulators, each of which corresponds to a channel, calculate the expected power of each channel signal. The base station also comprises a base station transceiver system controller (BTSC) which generates a desired output power of the sector by summing the expected powers of each channel.
A key aspect in implementing a wireless communication system is placement of antennas throughout the coverage area such that every location in the entire coverage area where a remote unit may be located is supported with sufficient signal levels. To create a distributed antenna, the transmit output of the base station is fed to a string of antenna elements each separated by delay. A distributed antenna exploits the ability of direct sequence CDMA to discriminate against multipath by creation of deliberate multipath that satisfies discrimination criteria.
A technique for improving performance of a distributed antenna system using parallel strings of discrete antennas wherein each antenna on a common string is separated from its neighbors by delay is disclosed in U.S. Pat. No. 5,280,472 entitled "CDMA MICROCELLULAR TELEPHONE SYSTEM AND DISTRIBUTED ANTENNA SYSTEM THEREFOR", which issued on Jan. 18, 1994 and assigned to the assignee of the present invention. Further development of the distributed antenna concept is disclosed in U.S. Pat. No. 5,513,176, issued on Apr. 30, 1996, entitled "DUAL DISTRIBUTED ANTENNA SYSTEM", and assigned to the assignee of the present invention. In the distributed antenna arrangement, signals transmitted from antennas of different antenna elements at a common node are provided different delay paths between the base station and the antenna. The antenna elements may comprise downconversion circuitry thus reducing the cabling path loss between the antenna elements and the base station and allowing the use of readily available SAW devices as delay elements.
Another advantage of the distributed antenna arrangement is that little site specific engineering is required for installation. Normally, antenna placement is determined only by physical constraints, together with the requirement that every location desiring service must be covered by a set of two antennas. There is no concern for the overlapping of antenna patterns. In fact, overlapping coverage is desirable in that it provides diversity operation to all terminals in the overlap area. Overlap is, however, not required.
An objective of a personal mobile communications network is to provide coverage over a large geographic region. Such broad geographic coverage is essential and must be provided on the first day of service to attract users in the present economic environment. One of the major expenses of providing coverage over a large geographic area is the acquisition of real estate and land usage rights and the installation of base stations each providing coverage for a portion of the total geographic coverage area.
Note that cable television (CATV) networks provide extensive coverage over nearly all suburban areas. Thus, if the CATV network, called the cable plant, could be used as the basis for a wireless communication network, the task of obtaining real estate and land usage rights and the expense of installing discrete base stations could be avoided. Thus, a centralized headend processor could provide the necessary signal processing functions at a single location within the geographic region and the cable distribution means could be used to carry the wireless signal to the users.
The characteristics of the CDMA system provide a myriad of advantages in a CATV based wireless system. The integration of the wireless communication network with the cable plant can be carefully orchestrated to take full benefit of the flexibility and capacity of the CDMA system. The present invention seeks to define such a system.